The present invention is directed to thyristor inverter circuits and, in particular, to commutation circuits for thyristors.
With the present interest in the use of variable frequency induction motors, state of the art inverters, whether they are thyristor or transistor based, are attracting attention. The thyristor inverter described in U.S. Pat. No. 3,207,974 which issued on Sept. 21, 1965 to W. McMurray, remains one of the most preferred, particularly in its modified form wherein a pair of diodes are connected across the commutation thyristors with a damping resistor connected between the juncture of the pair of diodes and the juncture of the commutation thyristors. This inverter is advantageous since it has a high efficiency, its commutation circuit components are small and it is suitable for pulse width modulation (PWM) operation. On the other hand, it has been found that this inverter has a propensity for misfirings and shoot-throughs and that protection against commutation failure or short circuit is difficult.
Power transistor inverters have emerged as a solution to many of these problems due to their control flexibility, however, power transistors cannot provide the power levels required in many applications.